1. Field of the Invention
The present invention relates to a camera and a program which improve accuracy of taken images by correcting image blur due to vibration.
2. Description of Related Art
Operations important for image-taking such as exposure determination and focusing, etc., of cameras that are currently used, have been completely automated, and even a person who is unskilled in camera operations is unlikely to cause an image-taking failure.
Furthermore, recently, a system that prevents vibration applied to the camera has also been studied, and factors that cause a photographer to fail in image taking have been reduced to almost zero.
Herein, a vibration isolation system that prevents vibration is briefly described.
For camera shake when image-taking appears as vibration with a frequency of, normally, 1 Hz through 10 Hz, and an essential consideration for enabling the camera to take a picture without image blur even when such vibration occurs at a point of exposure, it is required that camera shake due to vibration is detected and a correcting lens is displaced within an optical axis orthogonal plane according to the results of this detection (optical vibration isolation system).
Namely, in order to take a picture without image blur even when camera shake occurs, it becomes necessary that, first, camera vibration is accurately detected, and second, an optical axis change due to vibration is corrected.
Correction of image blur can be carried out by, in principle, mounting on a camera a vibration detecting part that detects acceleration, angular acceleration, angular velocity, and angular displacement by means of a laser gyro, etc., and carries out appropriate calculation for the results of this detection. Then, by driving a vibration correcting optical device (including a correcting lens) that makes an image-taking optical axis eccentric on the basis of the detection information on camera shake supplied from the vibration detecting part, image blur correction is carried out.
On the other hand, Japanese Patent No. 3110797 discloses a method in which image-taking is repeated a plurality of times in an exposure period with a length that does not cause vibration, and a plurality of images obtained through this image-taking are synthesized while correcting image divergence among the images to obtain a taken image (synthesized image) of a long exposure period.
Recent digital cameras have become smaller in size than silver-salt compact cameras, and in particular, a camera that has an image pickup device of a VGA class has been downsized so that it is built-in a portable electronics device (such as a portable phone).
Under these circumstances, in order to mount the abovementioned optical vibration isolation system on a camera, it is necessary that the vibration correcting optical device is made smaller or the vibration detecting part is made smaller.
However, in the vibration correcting optical device, since a correcting lens must be supported and highly accurately driven, there is a limit to downsizing. In addition, most of the vibration detecting parts that are currently used utilize inertia, so that if the vibration detecting parts are downsized, detection sensitivity lowers and accurate vibration correction becomes impossible.
Furthermore, vibration to be applied to cameras includes angular vibration around a predetermined axis and shifting vibration that shakes a camera parallel, and although the angular vibration is correctable by the optical vibration isolation system, the shifting vibration cannot be corrected by the optical vibration isolation system using inertia. Particularly, this shifting vibration tends to become greater as the camera becomes smaller.
On the other hand, as a different vibration isolation system, for being employed for taking a moving image, a method in which a motion vector of an image plane is detected based on an output of an image pickup device and an image readout position is changed according to the detected motion vector to obtain a moving image can also be employed.
Such a method has an advantage in that the entirety of the product can be downsized since the exclusive vibration detecting part and the correcting lens as in the abovementioned optical vibration isolation system become unnecessary.
However, this vibration isolation system for video cameras cannot be easily applied to digital cameras. The reason for this is described below.
Motion vector extraction in a video camera is carried out for each image reading, for example, when images of 15 frames are extracted per second, a motion vector is detected by comparing these extracted images.
However, in a case where a still image is taken by a digital camera, exposure is carried out only once for an object to be taken, so that detection of a motion vector through comparison of images as in a video camera is not possible.
Therefore, the vibration isolation system for video cameras cannot be simply applied to digital cameras.
On the other hand, in the vibration isolation method disclosed in Japanese Patent No. 3110797, since image-taking is repeated a plurality of times, an image-taking period becomes long. Therefore, if this method is recklessly used, it is feared that the number of failed pictures such as object vibration increase alternately.
In addition, a user must always note the use conditions of this vibration isolation method, and it becomes difficult to handle the system in comparison with a normal image-taking method in which image-taking is completed by one time of exposure. Hereinafter, this is described in detail.
Different from a silver-salt camera, in a digital camera, the sensitivity (imaging sensitivity) of the image pickup device can be freely changed. Therefore, even in a case of a dark object, by increasing the imaging sensitivity, vibration can be prevented to some extent.
Setting of the imaging sensitivity may be left to the camera (auto-setting) or may be set by a user himself/herself.
As mentioned above, in a digital camera, parameters that determine the necessity of use of vibration isolation are greater than in a silver-salt camera, so that it has become difficult for a user to judge whether or not a measure for vibration isolation is really necessary, and the system becomes difficult for users to handle.
Furthermore, in the abovementioned vibration isolation method, it is feared that a user experiences a sense of discomfort since exposure is carried out a plurality of times for one time of image taking.
Therefore, an object of the invention is to provide a small-sized vibration isolation system for still-image-taking of digital cameras different from the vibration isolation system for video cameras and the optical vibration isolation system for silver-salt cameras.
In addition, another object of the invention is to provide a camera in which, even when a method in which image-taking is repeated a plurality of times in an exposure period with a length that does not cause vibration and images obtained through the plurality of times of image-taking are synthesized while correcting divergence to obtain a taken image (synthesized image) of a long exposure period is mounted in this digital camera, the camera automatically sets this operation and prevents image-taking failures, and prevents a user from experiencing a sense of discomfort in handling this operation.